This invention relates to impellers of centrifugal blowers for use with air conditioning systems and other equipment, and, more particularly, to with an impeller of a unitary structure having blades of a configuration suitable for use with a centrifugal blower of low noise characteristic.
In, for example, U.S. Pat. No. 4,211,514, an impeller is proposed which includes a hub, a hub plate, a plurality of blades and a shroud.
Heretofore, various methods available for manufacturing the above described type of impeller have been proposed. In one method, the hub, hub plate and baldes are formed as a unitary structure, and the shroud is formed separately and joined to the unitary structure with, for example, a solvent. In another method, the shroud is mounted to a packaged unit of air conditioning system, for example, so that it will replace an impeller of the unitary structure. In still another method, the hub, hub plate and shroud are separately formed by pressing a sheet metal and joined to each other by spot welding, to assemble them together. Some disadvantages have been associated with these methods of the prior art for manufacturing impellers. The operation of assembling the parts, using a solvent or spot welding or the like, inevitably produces variations from one completed impeller to another, even if a worker strictly follows present procedures performing the manufacturing operation, rules governing the use of tools and materials, and number of inspections to be made as standards of operation. When the hub, hub plate and blades are formed as a unitary structure, and the shroud is separately formed and joined to the unitary structure, such method has a drawback which sufficient dimensional precisions of surfaces at which they are joined together cannot be obtained by shrinkage. This decreases the strength of the impeller thereby causing a reduction in reliability. To avoid this disadvantage, the impeller requires a machining operation to finish the dimensional precision thereof. The disadvantage noted hereinabove would become marked as the size of the impeller increases. When injection molding is used, it is necessary to employ a split mold for producing the blades which radially extend from the center of the impeller to its outer periphery. In molding the blades by using the split mold, it is necessary to remove the blades from the mold by starting with portions of the blades located at the outer periphery of the impeller and successively moving them vertically. This renders the construction of the mold complex, greatly increasing the cost of the mold.
An object of this invention is to provide an impeller of a centrifugal blower comprising a plurality of two-dimensional blades capable of being formed as a unitary structure by using a synthetic resin material and exhibiting a performance approximating that of three-dimensional blades.
Another object is to provide an impeller of a centrifugal blower exhibiting an improved efficiency which does not have variations in performance and strength.
One of the outstanding characteristics of the invention is the blades of two-dimensional profile of which the inner diameter on the hub plate side is smaller than on the shroud side and the radius of curvature of each blade at the blade inlet is successively reduced in going from the shroud toward the hub plate within the difference of the inner diameters.
Another outstanding characteristic is the blades of two-dimensional profile of which the outer diameter on the hub plate side is greater than on the shroud side and the curvature of each blade at the blade outlet is varied within the difference of the outer diameters so as to keep the blade outlet angle constant or nearly constant.
Still another outstanding characteristic is that the impeller can be formed integrally, so that there is no variation in performance and strength of the impeller. The hub, hub plate, blades and shroud are formed integrally, so that it is possible to eliminate the need to perform the operation of joining the parts together. This can reduce the cost. The outer diameter of the hub plate is smaller than the inner diameter of the shroud, and the parting plane of the molds is located in the blade section being within this difference of diameters. By this arrangement, the two molds used are of the most simple combination of movable and stationary molds which form a pair. This reduces the cost of mold. Another outstanding characteristic is the blade of the impeller of which thickness of the inlet side is slightly greater than that of the outlet side and the draft angles of the fluid pressure side and the back side of the blade in the same mold are different from each other and the angle provided at the inlet side of the blade is varied from that provided at the outlet side thereof, to thereby ensure that the inlet side of the fluid pressure side projects farther than the outlet side of the fluid pressure side at the main fluid flow line and that the outlet side of the backside projects farther than the inlet side of the backside with an obtuse angle. This arrangement enables a turbulence of airflow and the concentration of stresses to be avoided and makes it possible to form an impeller of a large diameter integrally.
Another outstanding characteristic is the impeller which comprises an integrally molded impeller assembly comprising a hub, a hub plate, a plurality of blades and shroud and a hub ring having an outer diameter greater than the inner diameter of the shroud and being secured to an outer periphery of the hub plate and end faces of the plurality of blades. The provision of the hub ring enables the mean outer diameter of the array of the blades to be increased to provide improvements in performance, so that the impeller provided with the hub ring can have high efficiency than the impeller having no hub ring even if the number of revolution is reduced. If the number of revolution can be reduced the impeller provided with the hub ring has increased strength because the stress is in proportion to the square of the number of revolution.